Membrane bioreactors (MBRs) are usually used in secondary treatment for removing COD, suspended solids (SS), and nitrogen-containing contaminants, etc. Due to the performance of biological treatment and membrane filtration, an MBR is capable of obtaining treated water quality that is close to a tertiary treatment level. Thus, an MBR is also suitable for use as a pre-treatment unit for wastewater reuse. At present, the more mature MBR units primarily belong to solid-liquid separation types, and usually use an ultrafiltration (UF) or micro-filtration (MF) membrane to concentrate biological sludge and obtain an effluent with excellent water quality.
The flux (m3/m2.day) of a membrane module is one of the most important control parameters for the MBR technique, and is related to the hydraulic retention time (HRT), volumetric loading, and transmembrane pressure of the membrane module. The flux of a membrane module is also an important factor in the installation and operating costs of a MBR system. In addition to SS characteristics and effluent quality, SS concentration has also effect on the flux of a membrane module. Although a higher concentration of SS indicates higher treatment efficiency, a higher concentration of sludge will also cause a reduction in the flux of an MBR, and an increase of the operating pressure thereof.
In US 2004-0079701 A1, the applicant of the present invention discloses a method of waste water/water treatment in a submerged membrane bioreactor. The bioreactor includes a tank containing a mixed liquor having microorganisms; porous carriers in the mixed liquors; and a filtration module immersed in the mixed liquor having a permeate side in fluid communication with the mixed liquor only through the filtration module. The method includes introducing an influent into the tank; applying a negative pressure source to the permeate side so that liquid in the tank penetrates the filtration module into the permeate side and flows out from the tank; and aerating the mixed liquor in the tank so that the porous carriers remain in contact with the filtration module, thus reducing fouling of the filtration module. Even though the above-mentioned patent application resolves a portion of the fouling problem, the application has its limitations. Since the concentrated sludge and the filtration module are located in the same reactor, it is difficult to maintain a high flux of water while simultaneously maintaining a high content of SS (a high concentration of microorganisms making degradation of contaminants easier). The disclosure in US 2004-0079701 A1 is incorporated herein by reference.